For the purpose of reinforcing a rubber to improve strength and durability thereof, a steel cord-rubber composite in which metal reinforcing materials such as steel cords are coated with a coating rubber are used for rubber articles required to have strength, such as tires for automobiles, conveyor belts, and hoses. Here, in order that such a steel cord-rubber composite can have a high reinforcing effect and reliability, stable and strong adhesion is required between the coating rubber and the metal reinforcing materials.
To provide such a steel cord-rubber composite having high adhesiveness between a coating rubber and metal reinforcing materials, so-called direct vulcanization adhesion is widely used. In the direct vulcanization adhesion, metal reinforcing materials such as steel cords that are plated with zinc, brass, and the like are embedded in a coating rubber comprising sulfur, and they are adhered to each other at the same time as the vulcanization of the rubber by heat vulcanization. Various studies on such direct vulcanization adhesion have been made to further improve adhesiveness between a coating rubber and metal reinforcing materials by the direct vulcanization adhesion.
For example, Patent Document 1 discloses a method of producing reinforcing steel cords for a steel cord-rubber composite formed by twisting a plurality of filaments manufactured by wet drawing of brass-plated steel wires. In this method, as an agent for improving adhesiveness between the steel cords and the coating rubber, resorcin is added to a wet drawing lubricant used in the drawing of the steel wires, so that resorcin is applied to the filament surfaces. Also, Patent Document 2 discloses a technique of cleaning surfaces of steel wires and steel cords to be used with an acidic or alkaline solution to remove a phosphorus compound (derived from a lubricant used for manufacturing the steel cords) which serves as an adhesion inhibitor, and thereby improving adhesion to the coating rubber.